1) Field of the Invention
The invention relates to a process for removing the sulphur compounds H.sub.2 S, SO.sub.2, COS and/or CS.sub.2 contained in a residual sulphur-plant gas, with the said compounds being recovered in the form of sulphur.
2) Background Art
Residual gases from sulphur plants, in which sulphur is produced by the Claus process of controlled oxidation of an acid gas containing H.sub.2 S by means of a gas containing free oxygen, contain of the order of 0.2% to 4% by volume of sulphur compounds, a large proportion of which consists of H.sub.2 S, the remainder consisting of SO.sub.2, COS, CS.sub.2 and sulphur vapour and/or vesicular sulphur.
Residual gases of this type are commonly treated to reduce their overall sulphur compound content as much as possible, in order to allow them to be discharged to the atmosphere after they have been incinerated, while meeting the standards imposed by law in terms of atmospheric pollution, and at the same time to recover these sulphur compounds in a form which contributes to increasing the yield of useful products formed from the acid gas treated in the sulphur plant.
Document EP-A-0346218 describes a process for removing the sulphur compounds H.sub.2 S, SO.sub.2, COS and/or CS.sub.2 contained in a residual sulphur-plant gas with the said compounds being recovered in the form of sulphur, which process consists (i) in bringing the residual gas from the sulphur plant into contact with a catalyst for the hydrolysis of the compounds COS and CS.sub.2 into H.sub.2 S, which is arranged in a hydrolysis unit, while operating at a temperature sufficient to produce a hydrolysed residual gas which contains H.sub.2 S and SO.sub.2 and is substantially free from COS and CS.sub.2, (ii) in conveying the hydrolysed residual gas, after a step of cooling to a temperature below the dew point of the sulphur in the gas, to a purification unit consisting of a plurality of catalytic zones, each containing a catalyst which promotes the Claus reaction, referred to as a Claus catalyst, and each operating in turn in reaction phase then in regeneration phase and finally in cooling phase so that, at any time, there is at least one catalytic zone in reaction phase and one catalytic zone in regeneration or cooling phase, (iii) in passing the hydrolysed residual gas conveyed to the purification unit through the catalytic zone or zones in reaction phase, while operating at temperatures below the dew point of sulphur, so that the compounds H.sub.2 S and SO.sub.2 present in the said gas react with one another to form sulphur which deposits on the Claus catalyst contained in the said zones, and so that a substantially purified residual gas is obtained at the outlet of the purification unit, (iv) in circulating a non-oxidizing gas stream having a temperature of between 200.degree. C. and 500.degree. C., more particularly between 250.degree. C. and 400.degree. C., through the catalytic zone in regeneration phase which contains Claus catalyst loaded with sulphur, in order to regenerate the said catalyst and form an effluent gas loaded with vaporized sulphur, and (v) in passing a gas stream having a temperature below 160.degree. C. through the catalytic zone containing the catalyst which has just been regenerated, in order to cool the said catalyst to the temperature required for again bringing it into contact with the hydrolysed residual gas.
In a variant of this process, as described in document EP-A-0424259, a gas flow containing free oxygen is introduced into the hydrolysis unit, and partial oxidation of H.sub.2 S into SO.sub.2, and optionally into sulphur, is carried out in the said hydrolysis unit by means of the said gas flow in contact with an H.sub.2 S oxidation catalyst contained in this unit, it being possible for the said catalyst to consist of the catalyst of the hydrolysis of the compounds COS and CS.sub.2 into H.sub.2 S, the flow rate of the said gas flow being controlled so as to keep the H.sub.2 S:SO.sub.2 molar ratio in the hydrolysed residual gas entering the purification unit at a value substantially equal to 2:1.
In another variant, which forms the subject-matter of the French Patent Application No. 9512988 filed on Mar. 11, 1995 in the name of the Applicant Company, and which is applicable both to the process in document EP-A-0346218 and to the process in document EP-A-0424259, the procedure adopted is such that the substantially purified residual gas leaving the purification unit contains H.sub.2 S as the only sulphur compound in a concentration of less than 0.5% by volume, and the said substantially purified residual gas is passed through a unit for catalytically oxidizing the residual H.sub.2 S into sulphur, which operates at a low temperature, that is to say at a temperature of below 150.degree. C., and for example ranging from 90.degree. C. to 130.degree. C., in order to complete the purification of the gas.
In the aforementioned process and its variants, the regeneration of the catalyst which is loaded with sulphur and the cooling of the regenerated catalyst are carried out by circulating the regeneration gas and the cooling gas in closed circuit by means of a blower, with heating and cooling equipment being used to bring the regeneration gas and the cooling gas to the appropriate temperatures.